Currently, the research with respect to three-dimensional surface profilometry and reconstruction are mostly based on three-dimensional optical measurement methodology, which can be classified into two main categories including direct optical triangulation and phase measurement methods. The direct optical triangulation utilizes the triangulation measurement principle for obtaining the depth information with respect to the surface profile of an object through a geometrical triangulation calculation basing upon data obtained by optical spot scanning in a point-to-point manner. Although the triangulation measurement is simple to implement, it is generally time consuming and not capable of achieving high measurement accuracy. On the other hand, the phase shifting measurement method being designed to measure phase information for surface heights by evaluating a deformed grating image resulting from the projection of structured fringe patterns upon the surface of an object is capable of obtaining the 3-D profile of the object through the mapping of the measured phases with the heights. Comparing with the direct triangulation, the phase measurement operation not only can be performed much faster and with higher accuracy, but also can achieve full field measurement.
There are two major categories for phase measurement method that are most commonly seen in the field of applications, which are Fourier transform technique and phase shifting method. The Fourier transform profilometry (FTP) had already been well established and known to those skilled in the art and thus will not be described further herein. Similarly, the phase shifting method had been vastly studied and used in 3-D surface profilometry. In general, the phase shifting method generates a wrapped phase map according to the phase measurement of deformed fringe patterns onto the surface of the object. Since the phase is wrapped in the inverse trigonometric functions, the Euler's equation and phase unwrapping operation are taken to obtain the phase information with respect to the height of the surface of the object and thereby reconstructing the corresponding surface profile of the object. It is noted that the aforesaid phase shifting profilometry is a fast full-field measurement of high accuracy.
In a prior art disclosed in “High-resolution Real-time 3-D shape Acquisition”, by Song Zhang and Peisen Huang at IEEE Computer Vision and Pattern Recognition Workshop, Vol. 03, No. 3, pp. 28-37, 2004, a high-resolution, real-time 3-D shape measurement system based on a digital fringe projection and phase-shifting technique is described. It utilizes a single-chip digital light processing projector to project three computer-generated fringe patterns having different phases that are shifted by 2π/3 from each other onto the object, and a high-speed CCD camera synchronized with the projector to acquire the fringe images at a frame rate of 120 frames/s. A color CCD camera is also used to capture images for surface texture mapping. Based on a three-step phase-shifting technique, each frame of the 3-D shape is reconstructed using the three consecutive fringe images. With the system, together with the fast three-step phase-shifting algorithm and parallel processing software being developed, a real-time 3-D shape measurement is realized.
In addition, there is another prior art disclosed in “Color-encoded Digital Fringe Projection Technique for High-speed Three-dimensional Surface Contouring” by Peisen Huang et al, at Optical Engineering (38), No. 6, 1999, pp. 1065-1071. In this technique, a color fringe pattern with a sinusoidal intensity profile whose RGB components comprising three 120-degree phase-shifted fringe patterns is created by computer software and then projected to an object by a computer-controlled digital projection system. The image of the object is captured by a digital camera positioned at an angle different from that of the projection system. The image is then separated into its RGB components, creating three phase-shifted images of the object. These three images are then used to retrieve the 3-D surface contour of the object through the use of a phase wrapping and unwrapping algorithm. Only one image of the object is required to obtain the 3-D surface contour of the object. Thus contouring speed, limited only by the frame rate of the camera, can be dramatically increased as compared to that of other traditional phase-shifting techniques.